This invention relates to an improved method for the production of an optical waveguide of the type disclosed in Bierlein et al U.S. Pat. Nos. 4,740,265 and 4,766,954.
In these patents a method is disclosed for producing an optical waveguide by contacting an optically smooth surface of a thin wafer cut from a crystal of K.sub.1-x Rb.sub.x TiOMO.sub.4 where 0.ltoreq.x.ltoreq.1 and M is P or As with a molten salt of at least one of Rb, Cs and T1. The wafer is cut so that its Z axis is perpendicular to the optically smooth surface. The crystal is treated with the molten salt at a temperature of about 200.degree. C. to about 600.degree. C. The crystal is treated with the molten salt for a time sufficient to increase the surface index of refraction at least about 0.00025 relative to the index of refraction of the starting crystal.
Particularly useful waveguides are those formed from crystals of KTiOPO.sub.4 hereinafter termed KTP.
In a high proportion of waveguides produced by the methods of the above-mentioned patents defects have been observed. This is shown by light scattering that frequently is observed during the testing of these waveguides.
It has been found that an important cause of these defects is mechanical damage to the optically smooth crystal surface that is present before treatment with the thallium salt. These are scratches or other surface damages resulting from cutting, lapping or polishing of the crystal.